If I start a side project, I find myself wanting to work on it instead of my daily job. But if I make the project my daily job, I want to work on something else. So no side projects for me, please. And I know plenty of other people who function like that.

Also, if you work hard on your job/project/startup for 7-8-9-10 hours a day, that's probably already past the point of diminishing returns.